1. Field of the Invention
The present invention belongs to a technological field related to magnetic resonance imaging systems for performing medical tomography by means of magnetic resonance phenomena, and particularly to a cosmetic cover for a superconductive electromagnetic device.
2. Description of the Related Art
Magnetic resonance imaging systems utilizing a superconductive electromagnetic device include tunnel-type and open-type systems when categorized according to the contours of vacuum containers for incorporating superconductive coils and the like. The vacuum container for a tunnel-type system has a cylindrical contour and a bore into which a subject is inserted. In contrast, the vacuum containers for an open-type system are in a top-and-bottom pair or in a left-and-right pair, facing each other, and a subject is inserted into the space between the facing vacuum containers.
In the open-type system, the two facing vacuum containers accommodate a subject between them, and the circumference of the subject-insertion space is not closed. As such, the open-type system is characterized by providing subjects with feeling of being not confined, whereby it has the advantage of providing claustrophobic patients and children with feeling of security. In addition, the open-type system has an advantage in terms of a subject-positioning work or of the application thereof to the IVR (Interventional Radiology) in which medical treatments are practiced with created images being observed.
The subject-insertion space of a current open-type system has a height of 38 through 55 cm. When this height is close to the lower limit (38 cm), even the open-type system could make the subjects feel being confined. The expansion of this height merely by several centimeters could abate the feeling of being confined of the subjects. Therefore, magnetic resonance imaging systems are required to eliminate the feeling of being confined of the subjects, by enlarging the subject-insertion space as large as possible.
In contrast, the subject-insertion bore of a current tunnel-type system has a diameter of about 60 through 70 cm. It goes without saying that this diameter (i.e., the subject-insertion space) is also required to be expanded as much as possible to eliminate the feeling of being confined of the subjects.
Meanwhile, a magnetic resonance imaging system is required to decrease magnetomotive force of its superconductive coils by bringing the superconductive coils as close as possible to a subject. This impairs feeling of not being confined of a subject. In other words, two contradictory things are required of a magnetic resonance imaging system, i.e., providing a subject with the feeling of not being confined and decreasing the magnetomotive force of the superconductive coils.
A first conventional magnetic resonance imaging system, in an open-type, has top and bottom static magnet field generating sources the respective total magnetic moments of which are asymmetric. In this way, the region of a uniform static magnetic field is shifted downwards, thereby enlarging the space over the subject (e.g., refer to International Publication No. 99/21476/panpflet). A second conventional magnetic resonance imaging system, in a tunnel-type, has gradient-magnetic-field coils the materials of which are asymmetrically arranged in the direction of the static magnetic field. In this way, the respective edges, at the head side of a subject, of the gradient-magnetic-field coils and radio frequency coils can be trued up, whereby the space over the head of the subject is enlarged, by devising a cosmetic cover at the head side of the subject (e.g., refer to Japanese Laid-Open Patent Publication 2000-102518).